One of the most important concepts in state-of-the-art video coding standards and algorithms is the so-called motion-compensated prediction. The pictures (or color components of pictures) of a video sequence are decomposed into regions. In general, these regions can have arbitrary shapes, but usually the regions represent quadratic or rectangular blocks. For the following description, only rectangular or quadratic blocks are considered; but the described concepts can be extended to arbitrary shaped regions in a straightforward way. The blocks of a picture can have variable sizes, i.e., different blocks in the same picture can have different sizes. The partitioning into blocks is often signaled inside the bitstream. For each block, it is decided whether it is intra-coded (i.e., without referring to already coded pictures in the video sequences) or whether it is inter-coded (i.e., with using already coded pictures for prediction). If a block is intra-coded, it is usually predicted using the reconstructed samples of already coded neighboring blocks inside the same picture. Inter-coded blocks are predicted using reconstructed samples of already coded pictures inside the video sequence. The prediction signal is generated by copying and potentially filtering the samples of one or more picture regions in already coded pictures. The one or more already coded pictures that are used for generating the inter-prediction signal, which are also referred to as reference pictures, are often signaled using so-called reference picture indices, which are either transmitted for each inter-coded block or are inferred (possibly based on certain high-level syntax elements). In addition, one or more motion parameter vectors are transmitted for a block, which specify the region in the reference pictures that are used for prediction and the filtering that is applied for generating the prediction signal. As a typical example, the motion parameters are represented by a displacement vector consisting of a horizontal and vertical component. The displacement vector components can have sub-sample accuracy. If the displacement vector has full-sample accuracy, it identifies a block in the reference picture, and the samples of this block are used as prediction signal for the current block, or the samples of the reference block are filtered to generate the prediction signal for the current block. In general, the displacement vectors have sub-sample accuracy, and then the reference blocks are additionally interpolated (depending on the sub-sample position) for generating the prediction signal for the current block. In this case, the displacement vector can also be interpreted as consisting of a full-sample part, which specifies the reference blocks, and a sub-sample part, which specifies the filtering of the reference block for generating the prediction signal. The motion parameters can also represent motion parameter vectors with more then two components. As an example, a higher order motion model as the affine motion model could be used for describing the motion of the current block between the reference picture and the current picture. In case of the affine motion model, a motion parameter vector consists of six components. But any other motion model with a particular number of motion parameter vector components could be employed. Since the prediction for inter-coded blocks is specified by a modeled motion of the current block relative to a reference picture, this type of prediction is often referred to as motion-compensated prediction. The final prediction signal for a block can also be generated by a superposition of two or more motion-compensated prediction signals. Each of the prediction signals is obtained as described above and the final prediction signal is generated by a weighted sum of the corresponding motion-compensated prediction signals. For both intra-coded and inter-coded blocks, the residual signal representing the difference between the original samples of a block and the samples of the prediction signal for a block is usually coded using transform coding. A two-dimensional transform is applied to the residual signal, the transform coefficients are quantized, and the resulting transform coefficient levels are entropy coded. The side information for a block, which may include the block partitioning information, the block coding modes (e.g., specifying whether the block is intra-coded or inter-coded and, if inter-coded, the number of motion-compensated prediction signals that are superposed), the intra-prediction modes, the reference picture indices, and the motion parameter vectors, is also included in the bitstream.
The side information rate related to motion parameter vectors can represent a significant amount of the overall bit rate. In order to reduce the side information rate, the motion parameter vectors of a current block are usually predicted using the motion parameter vectors of neighboring blocks of the current block. Only the differences between the predicted motion parameter vectors and the actual motion parameter vectors are transmitted.